An ultrasound signal is produced by exciting an ultrasonic transducer employed as a transmitter. Ultrasound is attenuated in the medium to be tested and/or is reflected at boundary surfaces. The ultrasound signal manipulated in this manner is subsequently received by an ultrasonic transducer employed as a receiver, the attenuation or reflection caused by the medium to be tested being detected, in addition to the transit time, by comparing the ultrasound signal transmitted and that received.
The measurement data obtained by the sensors are converted into electrical analogue signals and digitised in an analogue-to-digital converter to form digital ultrasound data. Said data contain the response signals which are produced in a through-transmission method or an echo method in response to an ultrasound signal and received by ultrasonic transducers. Examples of applications in which digitised ultrasound data are generated include medical ultrasound diagnosis and ultrasound-based material and component testing. In many of these ultrasound applications the data volume remains limited and it can therefore be stored without difficulty for subsequent offline evaluation. After being generated, the ultrasound data are passed on, either in real time or following intermediate storage, for subsequent, separate processing or use.
However, a significantly higher volume of ultrasound data is generated in complex ultrasound testing applications, for example tomography methods such as 3D ultrasound tomography methods (USCT) or ultrasound testing systems which operate independently over a longer period of time, for example crack or corrosion testing in pipes or pipelines. Processing said data in real-time, for example to produce tomography images, requires a very high degree of processing power. If it is not necessary to carry out a real-time evaluation, the data volumes must be stored in an intermediate manner in appropriately sized storage means.
“Testing pigs” which are provided with a large number of ultrasonic transducers (typically approximately 900 individual transducers) arranged circumferentially on the outer casing wall thereof are known, particularly in the field of pipeline testing, in particular oil or gas distribution pipelines. In order to examine the pipeline, the pig is inserted into the line and is transported through it along with the contents of the line. In this process, the condition of the pipeline is continuously examined by the transducers and the ultrasound data are stored inside the pig for subsequent evaluation. During the pig run through a long oil/gas line, very high data volumes accumulate owing to the large number of ultrasonic transducers on the one hand and the longer duration of the measurement sweeps through the pipeline on the other. In a pipeline test using a testing pig with 900 ultrasonic transducers, which operate at least in part in multiplex mode, at a test speed of 1 m/s (speed of the testing pig in the line), a data volume of approximately 900 TB accumulates over a pipeline distance of 500 km, at a data rate of approximately 2.8 GB/s. During a pig run of this type, this type of pig is not connected to the outside world. The data accumulated must therefore be stored in a form which allows the wall condition to be reconstructed outside the pipeline after the pig run, thus enabling the abnormalities/damage/defects in the pipe wall to be located and quantified reliably. Storing the data volumes accumulated does not appear practical or cost-effective, even with current storage media.
In medical sonography, in particular with complex applications such as tomography or high-resolution applications, very high data volumes also accumulate in a very short space of time, for example in the order of magnitude of 20 GB for medium-resolution tomographic mammography of a single breast. Although this very high data volume can be stored without difficulty in a stationary storage means, evaluating it without preselection requires very high and therefore expensive processing power on account of the high volume of raw data. To ensure that an image (reconstruction) can be produced in real time, the digital data obtained from the analogue values must be reduced/compressed.
In damage detection, data reduction methods therefore serve to extract the relevant features of a signal provided in connection with an anomaly or defect in the pipe wall or tissue and to reproduce them as accurately as possible in a minimum number of bits so as thereby to reduce/minimise the data volume to be stored.
It is essential to carry out data reduction to ensure that the data volume falls within a storable order of magnitude, that the run distance of the pig is cost-effective and that the intermediate storage means required for the testing pig or for ultrasound computed tomography is sufficient. Given that intermediate data storage prior to subsequent evaluation is not possible or is only possible with considerable storage space, in particular for independently operating systems such as the aforementioned pig systems, data reduction or data selection prior to storage is advisable.
Data reduction methods serve to extract all the features relevant for further processing from the ultrasound data and to save them in a reduced number of bits. It is possible to achieve considerably higher reduction factors through knowledge of the structure of the data and the weighting thereof, for example for the subsequent (offline) assessment of defects, by developing a specific reduction method adapted to the requirements of the signal evaluation.
One approach to this is disclosed in Barbian O. A., Grohs B., Licht R.: Signalanhebung durch Entstoerung von Laufzeit-Messwerten aus Ultarschallpruefungen von ferritischen und austenitischen Werkstoffen—ALOK, Part 1; Materialpruefung 23 (1981) 379-383. The method described in said document selects the peaks of an ultrasound signal envelope on the basis of the maximum values of half-waves. In a signal sequence comprising a number of repeating half-wave amplitudes of similar height, as measured in particular in the ultrasound echo method, selecting peak values in the assessment of the transit time is, however, subject to substantially greater variation in comparison with assessing the time from the slope of the envelope.
DE 40 40 190 A1 discloses a reduction method in which the time and amplitude are detected for the maximum value, for every digitally filtered reflected pulse. This method does not, however, evaluate the width and characteristic curve of the envelopes, and this leads to inaccuracies, in particular when assessing the pulse height. Moreover, the method requires an ultrasound signal smoothed by a low-pass filter.
DE 10 2005 005 386 B3 further discloses a method for specifically reducing digitised data from a large number of electromagnetic measurement transducers in an “EMAT pig” (electromagnetic acoustic transducer) to detect defects in sheet metal or pipes. The method is based on an algorithm for selecting maximum values from a sequence of peak values of the measured values and comprises an assessment of the size of a defect and of the signal background in the region surrounding a defect. The transit times must, however, be assessed accurately to compress the data of an ultrasound echo signal sequence with a plurality of peak values of virtually the same amplitude, and said method is too inaccurate.